The field of the present invention relates to a frequency synthesizer for RF pulses, an MRI apparatus and an RF pulse generating method. In more detail, it relates to a frequency synthesizer for RF pulses, an MRI apparatus and an RF pulse generating method permitting high speed switching-over of the frequency of the RF pulse under a high magnetostatic intensity.
An MRI apparatus is an apparatus that generates magnetic resonance signals by utilizing the magnetic resonance phenomenon, and thereby obtains tomograms of slices of the subject. An MRI apparatus selects the slice to be imaged by transmitting an RF pulse of a resonance frequency proportional to the magnetic field intensity of the slicing position. For an MRI apparatus, it is desirable to permit high speed changing of the slicing position to reduce the time taken to pick up an image, and accurate and quick switching-over of the frequency of the RF pulse is required.
Therefore, MRI apparatuses use a direct digital synthesizer (hereinafter abbreviated to DDS) to generate RF pulses (see, for instance, paragraph [0003] of Patent Document 1, FIG. 2). FIG. 10 is a block diagram showing the configuration of a conventional DDS 40. The DDS 40 is configured of a phase storage unit 41, a phase accumulator unit 42, a waveform table 43, a D/A converter 314 and a low-pass filter 44.
Phase increments are stored in the phase storage unit 41. Phase increments are inputted to the phase accumulator unit 42, and added to the accumulated phase in every clock period. The clock period here is the reciprocal of the sampling frequency fs. The accumulated phase is outputted from the phase accumulator unit 42, and the accumulated phase is inputted to the waveform table 43. The waveform table 43 is formed of, for instance, a ROM, and stores waveform values corresponding to accumulated phases. An accumulated phase is inputted to an address in the ROM, and a digital value of a waveform stored in the corresponding address is outputted from the ROM. The digital value of the waveform is inputted to the D/A converter 314, and converted into an analog waveform. Incidentally, signals from the phase storage unit 41 to the input of the D/A converter 314 are digital signals, and those from the output of the D/A converter 314 onward are analog signals.
FIG. 11 is a diagram showing the waveform of an analog signal outputted from the D/A converter when the digital value of a sine waveform is stored in the waveform table. As the output waveform of the D/A converter 314 is sampled in each clock period, it is stepwise as viewed in a time region and an alias signal (hereinafter it is referred to as alias in this specification, the scope of claims and drawings) is included, as viewed in terms of the frequency region. When the frequency of the input signal of the D/A converter 314 is fo, the alias appears in the frequency of n×fs±fo (n is a natural number). The alias is removed by the low-pass filter 44, and the analog waveform of the frequency fo is outputted from the DDS 40. Since the DDS 40 has no feedback loop such as a PLL (Phase Locked Loop), it can switch over the frequency at high speed.
FIG. 12 is a diagram showing the output of the D/A converter expressed in terms of the frequency region. When the frequency of the input signal of the D/A converter 314 is fo, the frequency of the first alias is fs−fo, and the frequencies of the second and third aliases are fs+fo and 2fs−fo, respectively. The output of the D/A converter 314, as viewed in terms of the frequency region, is attenuated along the slice of the sinc function of the next equation by the aperture effect (see, for instance, Non-Patent Document 1).
                              sin          ⁢                                          ⁢                      c            ⁡                          (                                                π                  ⁢                                                                          ⁢                  f                                fs                            )                                      =                              sin            ⁡                          (                                                π                  ⁢                                                                          ⁢                  f                                fs                            )                                                          π              ⁢                                                          ⁢              f                        fs                                              [                  Formula          ⁢                                          ⁢          1                ]            
The frequency fs/2, which is half the sampling frequency fs, is referred to as the Nyquist frequency. The range in which the attenuation of kept within −0.1 dB by the aperture effect is only up to the frequency of about 0.17 times the Nyquist frequency. As shown in FIG. 12, the magnitude of the output of the D/A converter 314 approaches zero in the vicinity of fs and in the vicinity of 2fs.
To cancel the influence of this aperture effect, a method that uses a pre-equalization filter whose frequency response is an inverse sinc function is proposed (see, for instance, Non-Patent Document 1). The inverse sinc function is a function represented by 1/sinc(x). By inputting signals increased in the amplitude of the high region by applying this pre-equalization filter to the D/A converter 314, the influence of the aperture effect can be cancelled.
FIG. 13 is a block diagram showing the configuration of a DDS that outputs aliases contained in the output of the D/A converter. A DDS 50 differs from the DDS 40 of FIG. 10 in that it uses a band-pass filter 315 instead of the low-pass filter 44. The phase storage unit 41, the phase accumulator unit 42, the waveform table 43 and the D/A converter 314 are common elements between the DDS 50 and the DDS 40. The band-pass filter 315 passes only specific aliases contained in the output of the D/A converter 314 (see, for instance, Patent Document 2). Even if the sampling frequency of the D/A converter 314 is low, an analog waveform output of a high frequency can be obtained.
Patent Document 1. Japanese Unexamined Patent Publication No. 2001-104281.
Patent Document 2 Japanese Unexamined Patent Publication No. Sho 63 (1988)-108807.
Non-Patent Document 1. http://www.ednjapan.com/content/issue/2006/07/content04.html, “To improve high frequency characteristics of D/A converters: Mechanism of deterioration of high frequency characteristics and three possible remedies”.
As increasing the intensity of the magnetostatic field provides a high S/N ratio, the resolution improvement of picked-up images and the shortening of the time taken to pick up images are thereby made possible. In recent years, the magnetic field intensities of MRI apparatuses have been increasingly enhanced, and a 3 T (tesla) ultra-high magnetic field MRI apparatus has been developed. In this 3 T ultra-high magnetic field MRI apparatus, the frequency of the RF pulses is as high as 128 MHz.
In order to generate RF pulses of a 128 MHz frequency required by the 3 T ultra-high magnetic field MRI apparatus by using a DDS 40 used for conventional MRI apparatuses, a D/A converter operating at a speed faster than twice 128 MHz is required. Furthermore in the farther future, the intensity of the magnetostatic field may reach or even surpass 4.7 T. In that case, much faster D/A converter would be needed. The operating speeds of the phase accumulator unit 42 and the waveform table 43 will also become faster, resulting increased power consumption.
Ultra-high frequency signals can be generated by using a PLL (Phase Locked Loop). However, as a PLL has a feedback loop, it involves difficulty in high speed switching-over of the frequency, and therefore cannot be considered most suitable for the generation of RF pulses.